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Introduced in 1994 as a complement for the M16, the US Carbine, Caliber 5.56mm, M4 was designed as an ideal rifle for close quarters battle, and was also made to be extremely modular with additional accessories. Due to this factor and the ever-increasing use for the size and maneuverability advantage, it displaced the M16 with many U.S. military units, although by 2014 the XM8 was used by Rat Patrol Team 01, the Army/Marine team at the Volta River, and later the troops on board the USS Missouri.

In 2014, the M4 Custom was one of the most flexible weapons at Old Snake's disposal, equipped with five attachment-mounting points that could be used to maximize its utility. The weapon was compatible with Snake's CQC, although it limited him to performing primarily throwdown and disarming maneuvers. Unlike the standard-issue M4, it had both 3-round burst and fully automatic firing modes in addition to semi-automatic fire. While there are a few AR-15 variants that use four firing positions, like some Navy M16A3 or the so-called "M4 Commando enhanced"[1], the M4 Custom only had a three-point selector switch. It's entirely possible that the M4 Custom doesn't have a safety at all, and is 'hot' at all times.[1]

The M4 Custom was given to Old Snake by Drebin 893, as an introductory "gift,". When Snake attempted to test the weapon, it was still locked, despite being laundered. At this time, Drebin realized that Snake's older generation of Nanomachines weren't compatible with the laundering process, so Snake was given an injection containing "suppressor nanomachines" in order to bypass this problem. Drebin also possessed at least two more M4 Customs inside his Stryker, presumably as weapons that were to be sold. In South America, Snake managed to get a clean headshot on Vamp using the M4, then briefly shot it out with the soldiers accompanying Vamp, before he and Naomi Hunter were rescued by Drebin. After the battle of the Metal Gears on Shadow Moses Island a wounded Snake attempted to shoot Liquid Ocelot with the M4 before collapsing from exhaustion. Snake later brought the M4 Custom aboard Outer Haven, temporarily dropping it to the floor as he began to suffer from seizures. Shortly afterwards, he used it to fend off Scarab unmanned weapons in Haven's server room, so that the Metal Gear Mk. III could upload the FOXALIVE virus into the GW AI.

During the early render of Metal Gear Solid 4, heavily customized M4 Customs acted as the standard issue weapon for most PMC soldiers, as trailers and images can be seen with soldiers using them against the Middle Eastern militiamen and Snake. The idea of the M4A1 being standard issue to the PMCs was later phased out in favor of the Mk.17 SCAR-H during the latter stages of development. The images of the PMCs using M4A1s can be seen in Act 1's briefing session, where Roy Campbell shows a few pictures in a slideshow while explaining the rapid growth of the PMCs. Drebin also described the M4 Custom to be one of the most favored weapons by the PMCs even though it is not seen being used in-game.

Old Snake always uses the M4 Custom's charging handle behind the rear sight to chamber new rounds, even though pressing the bolt release (the vertical button on the left side of the weapon) would perform the same function more quickly. This could be explained by the fact that some shooting schools believe that using the charging handle instead of the bolt release is more reliable (while technically unproven).

"An M4A1 with an improved kit designed with the Navy SEALs in mind. The kit was developed by the U.S. Naval Surface Warfare Center. The hand guard features a built-in rail system to accommodate various kinds of attachments -- laser illuminator, grenade launcher, vertical grip -- depending on the requirements of the mission."

―Weapon description in Metal Gear Solid: Portable Ops Plus.

The M4A1 (SOPMOD) in Metal Gear Solid: Portable Ops Plus.

The M4A1 goes under the name of M4 SOPMOD or M4A1 SOPMOD (Special Operations Peculiar Modification). In-game, it is only aesthetically different from Metal Gear Solid 4's M4 Custom and is equipped by Old Snake upon his recruitment.

^Metal Gear Solid 4: Guns of the Patriots, Kojima Productions (2008). The three-point selector switch is visible on the M4 Custom in-game. Given the nature of the weapon, its firing modes (semi-automatic, 3-round burst, and full automatic), it is entirely possible that Snake's M4 doesn't have a safety at all, and is "hot" at all times.